Polyareneazole/wood pulp and methods of making same

ABSTRACT

The present invention relates to wood pulp and polyareneazole pulp for use as reinforcement material in products including for example friction materials, fluid sealing materials, and papers. The pulp comprises (a) irregularly shaped, wood pulp fibrous structures, (b) irregularly shaped, polyareneazole fibrous structures, and (c) water, whereby wood pulp fibrils and/or stalks are substantially entangled with polyareneazole fibrils and/or stalks. The invention further relates to processes for making such wood pulp and polyareneazole pulp.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to wood pulp and polyareneazole pulp for use as areinforcement material in products including for example frictionmaterials, fluid sealing materials, and papers. The invention furtherrelates to processes for making such pulp.

2. Description of Related Art

Fibrous and non fibrous reinforcement materials have been used for manyyears in friction products, fluid sealing products and other plastic orrubber products. Such reinforcement materials typically must exhibithigh wear and heat resistance.

Asbestos fibers have historically been used as reinforcement materials,but due to their health risks, replacements have been made or proposed.However, many of these replacements do not perform as well as asbestosin one way or another.

Research Disclosure 74-75, published February 1980, discloses themanufacture of pulp made from fibrillated KEVLAR® brand para-aramidfibers of variable lengths and use of such pulp as a reinforcementmaterial in various applications. This publication discloses that pulpmade from KEVLAR® brand para-aramid fibers can be used in sheet productsalone, or in combination with fibers of other materials, such as NOMEX®brand meta-aramid, wood pulp, cotton and other natural cellulosics,rayon, polyester, polyolefin, nylon, polytetrafluoroethylene, asbestosand other minerals, fiberglass and other, ceramics, steel and othermetals, and carbon. The publication also discloses the use of pulp fromKEVLAR® brand para-aramid fiber alone, or with KEVLAR® brand para-aramidshort staple, in friction materials to replace a fraction of theasbestos volume, with the remainder of the asbestos volume beingreplaced by fillers or other fibers.

U.S. Patent Application Publication 2003/0022961 (to Kusaka et al.)discloses friction materials made from a friction modifier, a binder anda fibrous reinforcement made of a mixture of (a) a dry aramid pulp and(b) wet aramid pulp, wood pulp or acrylic fiber pulp. Dry aramid pulp isdefined as an aramid pulp obtained by “the dry fibrillation method”. Thedry fibrillation method is dry milling the aramid fibers between arotary cutter and a screen to prepare the pulp. Wet aramid pulp isdefined as an aramid pulp obtained by “the wet fibrillation method”. Thewet fibrillation method is milling short aramid fibers in water betweentwo rotary discs to form fibrillated fibers and then dehydrating thefibrillated fibers, i.e., the pulp. Kusaka et al further disclose amethod of mix-fibrillating fibers by first mixing plural types oforganic fibers that fibrillate at a definite ratio, and thenfibrillating the mixture to produce a pulp.

Polypyridobisimidazole polymer is a rigid rod polymer. Fiber made fromthis polymer (such as the polymer composition, which is referred to asPIPD and is known as the polymer used to make M5® fiber) is known to beuseful in both cut and flame resistant protective apparel. Rigid-rodpolymer fibers having strong hydrogen bonds between polymer chains,e.g., polypyridobisimidazoles, have been described in U.S. Pat. No.5,674,969 to Sikkema et al. An example of a polypyridobisimidazole ispoly(1,4-(2,5-dihydroxy)phenylene-2,6-pyrido[2,3-d:5,6-d′]bisimidazole),which can be prepared by the condensation polymerization oftetraaminopyridine and 2,5-dihydroxyterephthalic acid in polyphosphoricacid. Sikkema discloses that pulp can be made from these fibers. Sikkemaalso describes that in making one- or two-dimensional objects, such asfibers, films, tapes, and the like, it is desired thatpolypyridobisimidazoles have a high molecular weight corresponding to arelative viscosity (“Vrel” or “hrel”) of at least about 3.5, preferablyat least about 5, and more particularly equal to or higher than about10, when measured at a polymer concentration of 0.25 g/dl in methanesulfonic acid at 25° C. Sikkema also discloses that good fiber spinningresults are obtained withpoly[pyridobisimidazole-2,6-diyl(2,5-dihydroxy-p-phenylene)] havingrelative viscosities greater than about 12, and that relativeviscosities of over 50 (corresponding to inherent viscosities greaterthan about 15.6 dl/g) can be achieved.

There is an ongoing need to provide alternative pulps that both performwell in products and that are low in cost. Despite the numerousdisclosures proposing lower cost alternative reinforcement materials,many of these proposed products do not adequately perform in use, costsignificantly more than currently commercial products, or have othernegative attributes. As such, there remains a need for reinforcementmaterials that exhibit high wear and heat resistance, and that arecomparable or less expensive than other commercially availablereinforcement materials.

BRIEF SUMMARY OF THE INVENTION

One embodiment of this invention relates to a pulp for use asreinforcement or processing material, comprising:

-   -   (a) irregularly shaped, fibrillated wood pulp fibrous        structures, the structures being 60 to 97 weight percent of the        total solids;    -   (b) irregularly shaped, fibrillated polyarenazole fibrous        structures being 3 to 40 weight percent of the total solids; and    -   (c) water,

the wood pulp and the polyarenazole fibrous structures having an averagemaximum dimension of no more than 5 mm, a length-weighted average lengthof no more than 1.3 mm, and stalks and fibrils where the wood pulpfibrils and/or stalks are substantially entangled with the polyarenazolefibrils and/or stalks.

Another embodiment of this invention is a process for making afibrillated wood pulp and polyarenazole pulp for use as reinforcementmaterial, comprising:

(a) combining pulp ingredients including:

-   -   (1) wood pulp fiber having an average length of no more than 1        cm and being 60 to 97 weight percent of the total solids in the        ingredients;    -   (2) rigid rod polyarenazole fiber having an average length of no        more than 10 cm and being 3 to 40 weight percent of the total        solids in the ingredients; and    -   (3) water being 95 to 99 weight percent of the total        ingredients;

(b) mixing the ingredients to a substantially uniform slurry;

(c) co-refining the slurry by simultaneously:

-   -   (1) fibrillating, cutting and masticating the fibrillated wood        pulp fiber and the polyarenazole fiber to irregularly shaped        fibrillated fibrous structures with stalks and fibrils; and    -   (2) dispersing all solids such that the refined slurry is        substantially uniform; and

(d) removing water from the refined slurry,

thereby producing a fibrillated wood pulp and polyarenazole pulp withthe fibrillated wood pulp and the polyarenazole fibrous structureshaving an average maximum dimension of no more than 5 mm, alength-weighted average length of no more than 1.3 mm, and thefibrillated wood pulp fibrils and/or stalks are substantially entangledwith the polyarenazole fibrils and/or stalks.

Still another embodiment of this invention is a process for making anfibrillated wood pulp and polyarenazole pulp for use as reinforcementand processing material, comprising:

-   -   (a) combining ingredients including water and a first fiber from        the group consisting of:        -   (1) wood pulp fiber being 60 to 97 weight percent of the            total solids in the pulp; and        -   (2) rigid rod polyarenazole fiber being 3 to 40 weight            percent of the total solids in the pulp;    -   (b) mixing the combined ingredients to a substantially uniform        suspension;    -   (c) refining the suspension in a disc refiner thereby cutting        the fiber to have an average length of no more than 10 cm, and        fibrillating and masticating at least some of the fiber to        irregularly shaped fibrillated fibrous structures;    -   (d) combining ingredients including the refined suspension, the        second fiber of the group of (a)(1 and 2) having an average        length of no more than 10 cm, and water, if necessary, to        increase the water concentration to 95-99 weight percent of the        total ingredients;    -   (e) mixing the ingredients, if necessary, to form a        substantially uniform suspension;    -   (d) co-refining the mixed suspension by simultaneously:        -   (1) fibrillating, cutting and masticating solids in the            suspension such that all or substantially all of the wood            pulp and polyarenazole fiber is converted to irregularly            shaped fibrillated wood pulp and polyarenazole fibrous            structures with stalks and fibrils; and        -   (2) dispersing all solids such that the refined slurry is            substantially uniform; and    -   (f) removing water from the refined slurry,

thereby producing an wood pulp and polyarenazole pulp with thefibrillated wood pulp and the polyarenazole fibrous structures having anaverage maximum dimension of no more than 5 mm, a length-weightedaverage length of no more than 1.3 mm, and the wood pulp fibrils and/orstalks are substantially entangled with the polyarenazole fibrils and/orstalks.

In some embodiments this invention is further directed to frictionmaterials, fluid sealing materials, and papers comprising the pulp ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWING(S)

The invention can be more fully understood from the following detaileddescription thereof in connection with accompanying drawings describedas follows.

FIG. 1 is a block diagram of apparatus for performing a wet process formaking “wet” pulp in accordance with the present invention.

FIG. 2 is a block diagram of apparatus for performing a dry process formaking “dry” pulp in accordance with the present invention.

FIG. 3 is a digital optical micrograph of the prior art material that ismade when wood pulp is refined without any polyareneazole fiber beingpresent.

FIG. 4 is a digital optical micrograph of the fibrillation of PBO fiberafter refining.

FIG. 5 is a digital optical micrograph of the fibrillation of PBO andwood pulp after co-refining.

GLOSSARY

Before the invention is described, it is useful to define certain termsin the following glossary that will have the same meaning throughoutthis disclosure unless otherwise indicated.

“Fiber” means a relatively flexible, unit of matter having a high ratioof length to width across its cross-sectional area perpendicular to itslength. Herein, the term “fiber” is used interchangeably with the term“filament” or “end”. The cross section of the filaments described hereincan be any shape, but are typically circular or bean shaped. Fiber spunonto a bobbin in a package is referred to as continuous fiber orcontinuous filament or continuous filament yarns. Fiber can be cut intoshort lengths called staple fiber. Fiber can be cut into even smallerlengths called floc. Yarns, multifilament yarns or tows comprise aplurality of fibers. Yarn can be intertwined and/or twisted.

“Fibril” means a small fiber having a diameter as small as a fraction ofa micrometer to a few micrometers and having a length of from about 10to 100 micrometers. Fibrils generally extend from the main trunk of alarger fiber having a diameter of from 4 to 50 micrometers. Fibrils actas hooks or fasteners to ensnare and capture adjacent material. Somefibers fibrillate, but others do not or do not effectively fibrillateand for purposes of this definition such fibers do not fibrillate.

“Fibrillated fibrous structures” means particles of material having astalk and fibrils extending therefrom wherein the stalk is generallycolumnar and about 10 to 50 microns in diameter and the fibrils arehair-like members only a fraction of a micron or a few microns indiameter attached to the stalk and about 10 to 100 microns long.

“Floc” means short lengths of fiber, shorter than staple fiber. Thelength of floc is about 0.5 to about 15 mm and a diameter of 4 to 50micrometers, preferably having a length of 1 to 12 mm and a diameter of8 to 40 micrometers. Floc that is less than about 1 mm does not addsignificantly to the strength of the material in which it is used. Flocor fiber that is more than about 15 mm often does not function wellbecause the individual fibers may become entangled and cannot beadequately and uniformly distributed throughout the material or slurry.Aramid floc is made by cutting aramid fibers into short lengths withoutsignificant or any fibrillation, such as those prepared by processesdescribed in U.S. Pat. Nos. 3,063,966, 3,133,138, 3,767,756, and3,869,430. “Arithmetric” length means the calculated length from thefollowing formula:

${{Arithmetric}\mspace{14mu}{length}} = \frac{\sum\left\lbrack \left( {{Each}\mspace{14mu}{Individual}\mspace{14mu}{pulp}\mspace{14mu}{length}} \right) \right\rbrack}{\sum\left\lbrack {{Individual}\mspace{14mu}{pulp}\mspace{14mu}{count}} \right\rbrack}$

“Length-weighted average” length means the calculated length from thefollowing formula:

${{Length}\text{-}{weighted}\mspace{14mu}{average}\mspace{14mu}{length}} = \frac{\sum\left\lbrack \left( {{Each}\mspace{14mu}{Individual}\mspace{14mu}{pulp}\mspace{14mu}{length}} \right)^{2} \right\rbrack}{\sum\left\lbrack {{Each}\mspace{14mu}{Individual}\mspace{14mu}{pulp}\mspace{14mu}{length}} \right\rbrack}$

“Weight-weighted average” length means the calculated length from thefollowing formula:

${{Weight}\text{-}{weighted}\mspace{14mu}{average}\mspace{14mu}{length}} = \frac{\sum\left\lbrack \left( {{Each}\mspace{14mu}{Individual}\mspace{14mu}{pulp}\mspace{14mu}{length}} \right)^{3} \right\rbrack}{\sum\left\lbrack \left( {{Each}\mspace{14mu}{Individual}\mspace{14mu}{pulp}\mspace{14mu}{length}} \right)^{2} \right\rbrack}$

“Maximum dimension” of an object means the straight distance between thetwo most distal points from one another in the object

“Staple fiber” can be made by cutting filaments into lengths of no morethan 15 cm, preferably 3 to 15 cm; and most preferably 3 to 8 cm. Thestaple fiber can be straight (i.e., non crimped) or crimped to have asaw tooth shaped crimp along its length, with any crimp (or repeatingbend) frequency. The fibers can be present in uncoated, or coated, orotherwise pretreated (for example, pre-stretched or heat-treated) form.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to polyareneazole and wood pulp fiber pulpthat has use in friction materials, fluid sealing materials, and papers,and other materials that incorporate this pulp. The invention is alsodirected to processes for making a polyareneazole and wood pulp fiberpulp.

I. First Embodiment Of The Inventive Process

In a first embodiment, the process for making a wood pulp fiber andpolyareneazole pulp comprises the following steps. First, pulpingredients are combined, added or contacted together. Second, thecombined pulp ingredients are mixed to a substantially uniform slurry.Third, the slurry is simultaneously refined or co-refined. Fourth, wateris removed from the refined slurry.

Combining Step

In the combining step, the pulp ingredients are preferably addedtogether in a container. In a preferred embodiment the pulp ingredientsinclude (1) wood pulp fiber, (2) polyareneazole fiber, (3) optionallyother additives, and (4) water.

Wood Pulp Fiber

The wood pulp fiber is added to a concentration of 60 to 97 wt % of thetotal solids in the ingredients and preferably 60 to 75 wt % of thetotal solids in the ingredients.

The wood pulp fiber preferably has a coarseness of no more than 50 mgper 100 meters of length. In a preferred embodiment the coarseness isabout 12 to 25 mg per 100 meters of length. Fiber coarseness is definedas the mass of oven dried weight of pulp in mg divided by the totalcontour fiber length of all the fibers as measured using a FQA tabletopanalyzer Fiber Quality Analyzer (sold by OpTest Equipment Inc., 900Tupper St., Hawkesbury, ON, K6A 3S3 Canada)

In some embodiments, the wood pulp fiber has an average length of nomore than 1 cm. The wood pulp fiber preferably also has an averagelength of not more than about 5 mm.

“Wood pulp” as used herein refers to the product of boiling wood chipswith alkaline liquors or solutions of acidic or neutral salts followedby bleaching with chlorine compounds, the object being to remove more orless completely the hemicelluloses and lignin incrustants of the wood.Kraft pulp is a type of wood pulp and the method for making it involvescooking (digesting) wood chips in an alkaline solution for several hoursduring which time the chemicals attack the lignin in the wood. Thedissolved lignin is later removed leaving behind the cellulose fibers.Unbleached kraft pulp is dark brown in color, so before it can be usedin many papermaking applications it is typically bleached to lighten thecolor.

In some embodiments, the wood pulp of this invention includes lyocellfibers and other fibers or fibrous structures that are obtained fromcellulose with additional or different processing than described above

Polyareneazole Fiber

The polyareneazole fiber is added to a concentration of 3 to 40 wt % ofthe total solids in the ingredients, and preferably 25 to 40 wt % of thetotal solids in the ingredients. The polyareneazole fiber preferably hasa linear density of no more than 10 dtex and more preferably 0.8 to 2.5dtex. The polyareneazole fiber also preferably has an average lengthalong its longitudinal axis of no more than 10 cm, more preferably anaverage length of 0.65 to 2.5 cm, and most preferably an average lengthof 0.65 to 1.25 cm.

Polyarenazole Polymer

Polymers suitable for use in making the polyarenazole fiber must be offiber-forming molecular weight in order to be shaped into fibers. Thepolymers can include homopolymers, copolymers, and mixtures thereof

As defined herein, “polyareneazole” refers to polymers having either:

one heteroaromatic ring fused with an adjacent aromatic group (Ar) ofrepeating unit structure (a):

with N being a nitrogen atom and Z being a sulfur, oxygen, or NR groupwith R being hydrogen or a substituted or unsubstituted alkyl or arylattached to N; or two hetero aromatic rings each fused to a commonaromatic group (Ar¹) of either of the repeating unit structures (b1 orb2):

wherein N is a nitrogen atom and B is an oxygen, sulfur, or NR group,wherein R is hydrogen or a substituted or unsubstituted alkyl or arylattached to N. The number of repeating unit structures represented bystructures (a), (b1), and (b2) is not critical. Each polymer chaintypically has from about 10 to about 25,000 repeating units.Polyareneazole polymers include polybenzazole polymers and/orpolypyridazole polymers. In certain embodiments, the polybenzazolepolymers comprise polybenzimidazole or polybenzobisimidazole polymers.In certain other embodiments, the polypyridazole polymers comprisepolypyridobisimidazole or polypyridoimidazole polymers. In certainpreferred embodiments, the polymers are of a polybenzobisimidazole orpolypyridobisimidazole type.

In structure (b1) and (b2), Y is an aromatic, heteroaromatic, aliphaticgroup, or nil; preferably an aromatic group; more preferably asix-membered aromatic group of carbon atoms. Still more preferably, thesix-membered aromatic group of carbon atoms (Y) has para-orientedlinkages with two substituted hydroxyl groups; even more preferably2,5-dihydroxy-para-phenylene.

In structures (a), (b1), or (b2), Ar and Ar¹ each represent any aromaticor heteroaromatic group. The aromatic or heteroaromatic group can be afused or non-fused polycyclic system, but is preferably a singlesix-membered ring. More preferably, the Ar or Ar¹ group is preferablyheteroaromatic, wherein a nitrogen atom is substituted for one of thecarbon atoms of the ring system or Ar or Ar¹ may contain only carbonring atoms. Still more preferably, the Ar or Ar¹ group isheteroaromatic.

As herein defined, “polybenzazole” refers to polyareneazole polymerhaving repeating structure (a), (b1), or (b2) wherein the Ar or Ar¹group is a single six-membered aromatic ring of carbon atoms.Preferably, polybenzazoles include a class of rigid rod polybenzazoleshaving the structure (b1) or (b2); more preferably rigid rodpolybenzazoles having the structure (b1) or (b2) with a six-memberedcarbocyclic aromatic ring Ar¹. Such preferred polybenzazoles include,but are not limited to polybenzimidazoles (B═NR), polybenzthiazoles(B═S), polybenzoxazoles (B═O), and mixtures or copolymers thereof. Whenthe polybenzazole is a polybenzimidazole, preferably it ispoly(benzo[1,2-d:4,5-d′]bisimidazole-2,6-diyl-1,4-phenylene). When thepolybenzazole is a polybenzthiazole, preferably it ispoly(benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl-1,4-phenylene). When thepolybenzazole is a polybenzoxazole, preferably it ispoly(benzo[1,2-d:4,5-d′]bisoxazole-2,6-diyl-1,4-phenylene).

As herein defined, “polypyridazole” refers to polyareneazole polymerhaving repeating structure (a), (b1), or (b2) wherein the Ar or Ar¹group is a single six-membered aromatic ring of five carbon atoms andone nitrogen atom. Preferably, these polypyridazoles include a class ofrigid rod polypyridazoles having the structure (b1) or (b2), morepreferably rigid rod polypyridazoles having the structure (b1) or (b2)with a six-membered heterocyclic aromatic ring Ar¹. Such more preferredpolypyridazoles include, but are not limited to polypyridobisimidazole(B═NR), polypyridobisthiazole (B═S), polypyridobisoxazole (B═O), andmixtures or copolymers thereof. Yet more preferred, the polypyridazoleis a polypyridobisimidazole (B═NR) of structure:

wherein N is a nitrogen atom and R is hydrogen or a substituted orunsubstituted alkyl or aryl attached to N, preferably wherein R is H.The average number of repeat units of the polymer chains is typically inthe range of from about from about 10 to about 25,000, more typically inthe range of from about 100 to 1,000, even more typically in the rangeof from about 125 to 500, and further typically in the range of fromabout 150 to 300.

For the purposes of this invention, the relative molecular weights ofthe polyareneazole polymers are suitably characterized by diluting thepolymer products with a suitable solvent, such as methane sulfonic acid,to a polymer concentration of 0.05 g/dl, and measuring one or moredilute solution viscosity values at 30° C. Molecular weight developmentof polyareneazole polymers of the present invention is suitablymonitored by, and correlated to, one or more dilute solution viscositymeasurements. Accordingly, dilute solution measurements of the relativeviscosity (“Vrel” or “hrel” or “nrel”) and inherent viscosity (“Vinh” or“hinh” or “ninh”) are typically used for monitoring polymer molecularweight. The relative and inherent viscosities of dilute polymersolutions are related according to the expressionVinh=ln(Vrel)/C,where ln is the natural logarithm function and C is the concentration ofthe polymer solution. Vrel is a unitless ratio of the polymer solutionviscosity to that of the solvent free of polymer, thus Vinh is expressedin units of inverse concentration, typically as deciliters per gram(“dl/g”). Accordingly, in certain aspects of the present invention thepolypyridoimidazole polymers are produced that are characterized asproviding a polymer solution having an inherent viscosity of at leastabout 20 dl/g at 30° C. at a polymer concentration of 0.05 g/dl inmethane sulfonic acid. Because the higher molecular weight polymers thatresult from the invention disclosed herein give rise to viscous polymersolutions, a concentration of about 0.05 g/dl polymer in methanesulfonic acid is useful for measuring inherent viscosities in areasonable amount of time.

In some embodiments, this invention utilizes polyareneazole fiber thathas an inherent viscosity of at least 20 dl/g; in other more preferredembodiments the inherent viscosity is of at least 25 dl/g; and in somemost preferred embodiments the inherent viscosity is of at least 28dl/g.

Optional Other Additives

Other additives can optionally be added as long as they stay suspendedin the slurry in the mixing step and do not significantly change theeffect of the refining step on the mandatory solid ingredients listedabove. Suitable additives include pigments, dyes, anti-oxidants,flame-retardant compounds, and other processing and dispersing aids.Preferably, the pulp ingredients do not include asbestos. In otherwords, the resulting pulp is asbestos free or without asbestos.

Water

Water is added to a concentration of 95 to 99 wt % of the totalingredients, and preferably 97 to 99 wt % of the total ingredients.Further, the water can be added first. Then other ingredients can beadded at a rate to optimize dispersion in the water while simultaneouslymixing the combined ingredients.

Mixing Step

In the mixing step, the ingredients are mixed to a substantially uniformslurry. By “substantially uniform” is meant that random samples of theslurry contain the same wt % of the concentration of each of thestarting ingredients as in the total ingredients in the combination stepplus or minus 10 wt %, preferably 5 wt % and most preferably 2 wt %. Forinstance, if the concentration of the solids in the total mixture is 50wt % wood pulp fiber plus 50 wt % polyareneazole fiber, then asubstantially uniform mixture in the mixing step means each randomsample of the slurry has (1) a concentration of the wood pulp fiber of50 wt % plus or minus 10 wt %, preferably 5 wt % and most preferably 2wt % and (2) a concentration of polyareneazole fiber of 50 wt % plus orminus 10 wt %, preferably 5 wt % and most preferably 2 wt %. The mixingcan be accomplished in any vessel containing rotating blades or someother agitator. The mixing can occur after the ingredients are added orwhile the ingredients are being added or combined.

Refining Step

In the refining step the pulp ingredients are simultaneously co-refined,converted or modified as follows. The wood pulp fiber and thepolyareneazole fiber are fibrillated, cut and masticated to irregularlyshaped fibrous structures having stalks and fibrils. All solids aredispersed such that the refined slurry is substantially uniform.“Substantially uniform” is as defined above. The refining steppreferably comprises passing the mixed slurry through one or more discrefiner, or recycling the slurry back through a single refiner. By theterm “disc refiner” is meant a refiner containing one or more pair ofdiscs that rotate with respect to each other thereby refiningingredients by the shear action between the discs. In one suitable typeof disc refiner, the slurry being refined is pumped between closelyspaced circular rotor and stator discs rotatable with respect to oneanother. Each disc has a surface, facing the other disc, with at leastpartially radially extending surface grooves. A preferred disc refinerthat can be used is disclosed in U.S. Pat. No. 4,472,241. In a preferredembodiment, the plate gap setting for the disc refiner is a maximum of0.18 mm and preferably the gap setting is 0.13 mm or lower, to apractical minimum setting of about 0.05 mm.

If necessary for uniform dispersion and adequate refining, the mixedslurry can be passed through the disc refiner more than once or througha series of at least two disc refiners. When the mixed slurry is refinedin only one refiner, there is a tendency for the resulting slurry to beinadequately refined and non uniformly dispersed. Conglomerates oraggregates entirely or substantially of one solid ingredient, or theother, or both, can form rather than being dispersed forming asubstantially uniform dispersion. Such conglomerates or aggregates havea greater tendency to be broken apart and dispersed in the slurry whenthe mixed slurry is passed through the refiner more than once or passedthrough more than one refiner. Optionally, the refined slurry may bepassed through a screen to segregate long fibers or clumps, which may berecycled through one or more refiners until cut to acceptable lengths orconcentration.

Because a substantially uniform slurry containing multiple ingredientsis co-refined in this step of the process, any one type of pulpingredient (for example, polyareneazole fiber) is refined into a pulp inthe presence of all the other types of pulp ingredients (for example,wood pulp fiber) while those other ingredients are also being refined.This co-refining of pulp ingredients forms a pulp that is superior to apulp blend generated by merely mixing two pulps together. Adding twopulps and then merely mixing them together does not form thesubstantially uniform and intimately connected fibrous components of thepulp generated by co-refining of pulp ingredients into pulp inaccordance with the present invention.

Removing Step

Then water is removed from the refined slurry. The water can be removedby collecting the pulp on a dewatering device such as a horizontalfilter, and if desired, additional water can be removed by applyingpressure or squeezing the pulp filter cake. The dewatered pulp canoptionally then be dried to a desired moisture content, and/or can bepackaged or wound up on rolls. In some preferred embodiments, the wateris removed to a degree that the resulting pulp can be collected on ascreen and wound up into rolls. In some embodiments, no more than about60 total wt % water being present is a desired amount of water andpreferably 4 to 60 total wt % water. However, in some embodiments, thepulp can retain more water, so higher amounts of total water, as much as75 total wt % water, will be present.

FIGS. 1 and 2

This process will now be described in reference to FIGS. 1 and 2.Throughout this detailed description, similar reference characters referto similar elements in all figures of the drawings.

Referring to FIG. 1, there is a block diagram of an embodiment of a wetprocess for making “wet” pulp in accordance with the present invention.Pulp ingredients 1 are added to container 2. Container 2 is providedwith an internal mixer, similar to a mixer in a washing machine. Themixer disperses the ingredients into the water creating thesubstantially uniform slurry. The mixed slurry is transferred to a firstrefiner 3 that refines the slurry. Then, optionally, the refined slurrycan be transferred to a second refiner 4, and optionally then to a thirdrefiner 5. Three refiners are illustrated but any number of refiners canbe used depending on the degree of uniformity and refining desired.After the last refiner in the series of refiners, the refined slurry isoptionally transferred to a filter or sorter 6 that allows slurry withdispersed solids below a chosen mesh or screen size to pass andrecirculates dispersed solids larger than a chosen mesh or screen sizeback to one or more of the refiners such as through line 7 or to arefiner 8 dedicated to refine this recirculated slurry from whichrefined slurry is again passed to the filter or sorter 6. Suitablyrefined slurry passes from the filter or sorter 6 to a horizontal watervacuum filter 9 that removes water. Slurry can be transferred from pointto point by any conventional method and apparatus such as with theassistance of one or more pump 10. Then the pulp is conveyed to a dryer11 that removes more water until the pulp has the desired concentrationof water. Then the refined pulp is packaged in a baler 12.

Referring to FIG. 2, there is a block diagram of an embodiment of a dryprocess for making “dry” pulp in accordance with the present invention.This dry process is the same as the wet process except after thehorizontal water vacuum filter 9. After that filter, the pulp goesthrough a press 13 that removes more water. Then the pulp goes through afluffer 14 to fluff the pulp and then a dryer 11 to remove more water tothe desired concentration. Then the pulp is passed through a rotor 15and packaged in a baler 12.

II. Second Embodiment Of The Inventive Process

In a second embodiment, the process for making the wood pulp andpolyareneazole pulp is the same as the first embodiment of the processdescribed above with the following differences.

Prior to combining all ingredients together, either the wood pulp fiberor the polyareneazole fiber, or both the wood pulp fiber and thepolyareneazole fiber, may need to be shortened. This is done bycombining water with the fiber ingredient. Then the water and fiber aremixed to form a first suspension and processed through a first discrefiner to shorten the fiber. The disc refiner cuts the fiber to anaverage length of no more than 10 cm. The disc refiner will alsopartially fibrillate and partially masticate the fiber. The other fiber,that was not previously added, can be shortened this way too forming asecond processed suspension. Then the other fiber (or the secondsuspension, if processed in water) is combined with the firstsuspension.

More water is added before or after, or when, other ingredients areadded, if necessary, to increase the water concentration to 95-99 wt %of the total ingredients. After all ingredients are combined, they canbe mixed, if necessary, to achieve a substantially uniform slurry.

The ingredients in the slurry are then co-refined together, i.e.,simultaneously. This refining step includes fibrillating, cutting andmasticating solids in the suspension such that all or substantially allof the wood pulp fiber and polyareneazole fiber is converted toirregularly shaped fibrillated fibrous structures. This refining stepalso disperses all solids such that the refined slurry is substantiallyuniform. Then water is removed as in the first embodiment of theprocess. Both processes produce the same or substantially the same woodpulp fiber and polyareneazole pulp.

The Inventive Pulp

The resulting product produced by the process of this invention is awood pulp and polyareneazole pulp for use as reinforcement material infriction and fluid sealing products and papers. The pulp comprises (a)irregularly shaped, wood pulp fibrous structures, (b) irregularlyshaped, polyareneazole fibrous structures, (c) optionally other minoradditives, and (d) water.

The concentration of the separate ingredient components in the pulpcorrespond, of course, to the concentrations described beforehand of thecorresponding ingredients used in making the pulp.

The irregularly shaped, wood pulp and polyareneazole fibrillated fibrousstructures have stalks and fibrils. The wood pulp fibrils and/or stalksare substantially entangled with the polyareneazole fibrils and/orstalks. The fibrils are important and act as hooks or fasteners ortentacles that adhere to and hold adjacent particles in the pulp andfinal product thereby providing integrity to the final product.

The wood pulp and polyareneazole fibrillated fibrous structurespreferably have an average maximum dimension of no more than 5 mm, morepreferably 0.1 to 4 mm, and most preferably 0.1 to 3 mm. The wood pulpand polyareneazole fibrillated fibrous structures preferably have alength-weighted average of no more than 1.3 mm, more preferably 0.7 to1.2 mm, and most preferably 0.75 to 1.1 mm.

In a preferred embodiment, the wood pulp and polyareneazole pulp iswithout substantial aggregates or conglomerates of the same material.Further, the pulp has a Canadian Standard Freeness (CSF) as measured perTAPPI test T 227 om-92, which is a measure of its drainagecharacteristics, of 100 to 700 ml, and preferably 250 to 450 ml.

Surface area of pulp is a measure of the degree of fibrillation andinfluences the porosity of the product made from the pulp. In someembodiments of this invention, the surface area of the pulp is 7 to 11square meters per gram.

It is believed that the fibrillated fibrous structures, dispersedsubstantially homogeneously throughout the reinforcement material, andthe friction and fluid sealing materials, provide, by virtue of the hightemperature characteristics of the polyareneazole polymers and thefibrillation propensity of the polyareneazole fibers, many sites ofreinforcement and increased wear resistance. When co-refined, theblending of the wood pulp and polyareneazole materials is so intimatethat in a friction or fluid sealing material there is always somepolyareneazole fibrous structures close to the wood pulp fiberstructures, so the stresses and abrasion of service are always shared.Therefore, when co-refined, the wood pulp and the polyareneazolematerials are in such intimate contact that in a friction or fluidsealing material there are always some polyareneazole fibrous structuresclose to the wood pulp fiber structures so the stresses and abrasion ofservice are always shared.

Friction Material

The pulp of the present invention can be used as a reinforcementmaterial in friction materials. By “friction materials” is meantmaterials used for their frictional characteristics, such as coefficientof friction, to stop or transfer energy of motion, stability at hightemperatures, wear resistance, noise and vibration damping properties,etc. Illustrative uses for friction materials include brake pads, brakeblocks, dry clutch facings, clutch face segments, brake padbacking/insulating layers, automatic transmission papers, wet brake, andother industrial friction papers.

In view of this new use, the invention is further directed to frictionmaterial and processes for making the friction material. Specifically,the friction material comprises a friction modifier; optionally at leastone filler; a binder; and a fibrous reinforcement material comprisingthe wood pulp and polyareneazole pulp of this invention. Suitablefriction modifiers are metal powders such as iron, copper and zinc;abrasives such as oxides of magnesium and aluminum; lubricants, such assynthetic and natural graphites, and sulfides of molybdenum andzirconium; and organic friction modifiers such as synthetic rubbers andcashew nut shell resin particles. Suitable binders are thermosettingresins such as phenolic resins (i.e., straight (100%) phenolic resin andvarious phenolic resins modified with rubber or epoxy), melamine resins,epoxy resins and polyimide resins, and mixtures thereof. Suitablefillers include barite, whiting, limestone, clay, talc, various othermagnesium-aluminum-silicate powders, wollastonite, attapulgite, andmixtures thereof.

The actual steps for making the friction material can vary, depending onthe type of friction material desired. For example, methods for makingmolded friction parts generally involve combining the desiredingredients in a mold, curing the part, and shaping, heat treating andgrinding the part if desired. Automotive transmission and frictionpapers generally can be made by combining the desired ingredients in aslurry and making a paper on a paper machine using conventional papermaking processes.

Fluid Sealing Material

The invention is further directed to fluid sealing material andprocesses for making the fluid sealing materials. Fluid sealingmaterials are used in or as a barrier to prevent the discharge of fluidsand/or gases and used to prevent the entrance of contaminants where twoitems are joined together. An illustrative use for fluid sealingmaterial is in gaskets. The fluid sealing material comprises a binder;optionally at least one filler; and a fibrous reinforcement materialcomprising the wood pulp and polyareneazole pulp of this invention.Suitable binders include nitrile rubber, butadiene rubber, neoprene,styrene-butadiene rubber, nitrile-butadiene rubber, and mixturesthereof. The binder can be added with all other starting materials. Thebinder is typically added in the first step of the gasket productionprocess, in which the dry ingredients are mixed together. Otheringredients optionally include uncured rubber particles and a rubbersolvent, or a solution of rubber in solvent, to cause the binder to coatsurfaces of the fillers and pulp. Suitable fillers include bariumsulfate, clays, talc, and mixtures thereof.

Suitable processes for making fluid sealing materials are, for example,a beater-add process or wet process where the gasket is made from aslurry of materials, or by what is called a calendaring or dry processwhere the ingredients are combined in an elastomeric or rubber solution.

Many other applications of the pulp are possible, including its use as acomponent in papers, or its use as a filter material. When used as apaper or filter material typically the pulp of this invention iscombined with a binder and a molded part or sheet or paper product ismade by conventional methods.

Test Methods

The following test methods were used in the following Examples.

Canadian Standard Freeness (CSF) was measured as described in TAPPImethod T 227 in conjunction with optical microscopy. CSF measures thedrainage rate of a dilute pulp suspension. It is a useful test to assessthe degree of fibrillation. Data obtained from conduct of that test areexpressed as Canadian Freeness Numbers, which represent the millilitersof water that drain from an aqueous slurry under specified conditions. Alarge number indicates a high freeness and a high tendency for water todrain. A low number indicates a tendency for the dispersion to drainslowly. The freeness is inversely related to the degree of fibrillationof the pulp, since greater numbers of fibrils reduce the rate at whichwater drains through a forming paper mat.

Average fiber lengths, including Length-weighted average length, weredetermined using a Fiber Quality Analyzer (sold by OpTest EquipmentInc., 900 Tupper St., Hawkesbury, ON, K6A 3S3 Canada) following TAPPItest method T 271.

Temperature: All temperatures are measured in degrees Celsius (° C.).

Denier is measured according to ASTM D 1577 and is the linear density ofa fiber as expressed as weight in grams of 9000 meters of fiber. Thedenier is measured on a Vibroscope from Textechno of Munich, Germany.Denier times (10/9) is equal to decitex (dtex).

EXAMPLES

This invention will now be illustrated by the following specificexamples. All parts and percentages are by weight unless otherwiseindicated. Examples prepared according to the process or processes ofthe current invention are indicated by numerical values. Comparativeexamples are indicated by letters.

The following examples illustrate the surprising increase in the degreeof fibrillation of a wood pulp fiber by co-refining a small amount ofpolyarenazole fiber in the presence of the wood pulp fiber. The degreeof fibrillation is an important characteristic of a pulp product. Thereis a direct relationship between degree of fibrillation and fillerretention. In addition, fibrillation is useful to achieve uniformdispersion of the pulp products in a variety of materials. A highlyfibrillated fiber will also be able to bond to a matrix more intenselythrough physical entanglement than a non-fibrillated fiber. In theexamples that follow, poly(paraphenylene benzobisoxazole) (PBO) fiberwas used as a representative of the polyarenazole fiber family and kraftpulp was used to represent wood pulp fibers.

Comparative Example A

This example illustrates prior art material that is made when wood pulpfiber is refined without any polyarenazole fiber being present.

68.1 grams of a hardwood pulp (Hawesville Hardwood, bleached Kraft pulp,sold by Weyerhaeuser Company PO Box 9777 Federal Way, Wash. 98063-9777)was dispersed in 2.7 L of water. The dispersion was passed 5 timesthrough a Sprout-Wadron single-speed, 30 cm single disk refiner (sold byAndritz, Inc., Sprout-Bauer Equipment, Muncy, Pa. 17756) with the diskgap set to 0.13 mm. The properties of the as-produced 100% refinedwoodpulp are shown in Table 1; FIG. 3 is a digital optical micrograph ofthe material showing the limited fibrillation experienced by thismaterial after refining.

A paper was then made from the refined material by dispersing with astandard pulp disintegrator (as described in Appendix A of TAPPI 205)6.7 grams of the material (on a dry weight basis) in 1.5 L water for 3min, adding the dispersion to a wet-laid paper mold having a screen withthe dimensions of 21 cm×21 cm. The dispersion was then diluted with 5 Lof water and a wet-laid paper was formed on the screen and excess waterwas removed with a rolling pin. The paper was then dried at 100° C. for10 min in a paper dryer. The properties of the as-produced paper areshown in Table 2.

Comparative Example B

This example illustrates a 100% polyarenazole pulp. A 100% PBO pulp wasproduced using the same procedure as in Comparative Example A with theexception of using 68.1 grams of a 1.7 dtex PBO fiber having a cutlength of 12.7 mm (sold by Toyobo Co., Ltd., Zylon Department, 2-2-8Dojima-Hama, Kita-Ku Osaka) rather than wood pulp. The properties of theas-produced 100% PBO refined material are shown in Table 1; FIG. 4 is adigital optical micrograph of the pulp showing the fibrillation of thePBO fiber after refining. A paper was then made (as described inComparative Example A) from the PBO refined material and properties ofthe as-produced paper are shown in Table 2.

Example 1

A pulp of this invention was produced using the same procedure as inComparative Example A with the exception a dispersion containing amixture of the starting unrefined cut fibers of Comparative Example Aand the starting unrefined cut fibers of Comparative Example B wasrefined, passing 17 times through the disk refiner to form a co-refinedpulp. The fiber mixture contained 61.7 grams of a hardwood pulp(Hawesville Hardwood, bleached Kraft pulp, sold by Weyerhaeuser CompanyPO Box 9777 Federal Way, Wash. 98063-9777) and 6.4 grams of 1.7 dtex PBOfiber having a cut length of 12.7 mm (sold by Toyobo Co., Ltd., ZylonDepartment, 2-2-8 Dojima-Hama, Kita-Ku Osaka). Properties of theas-produced pulp are shown in Table 1. A paper was then made (asdescribed in Comparative Example A) from the pulp and properties of theas-produced paper are shown in Table 2.

Example 2

Another pulp of this invention was produced using the same procedure asin Example 1 with the exception the mixture contained 50.8 grams of thehardwood pulp and 17.3 grams of the 1.7 dtex PBO fiber. The co-refinedpulp had approximately 25 weight percent PBO and 75 weight percent woodpulp. The properties of the as-produced pulp are shown in Table 1; FIG.5 is a digital optical micrograph of the pulp showing the fibrillationof both the PBO and wood pulp fiber after refining. A paper was thenmade (as described in Comparative Example A) from the pulp andproperties of the as-produced paper are shown in Table 2.

Comparative Example C

This example demonstrates that refining the wood pulp fibers separatelyfrom the polyareneazole fibers and then mixing them together results ina pulp that provides a paper having lower tensile strength (andtherefore less fibrillation) than a paper made from the co-refined pulpof this invention.

A sample of the refined material made in Comparative Example A was mixedwith a sample of the refined material of Comparative Example B in anamount of 75 wt % wood pulp material to 25% wt % PBO material (dryweight basis) using a standard disintegrator as described in Appendix Aof TAPI 205 for 5 min. The TAPPI disintegrator was used to mix the tworefined pulps of Comparative Examples A and B because the agitation isvigorous enough to mix and disperse the previously refined pulps well,but it would not change their length or fibrillation. The properties ofthe as-produced pulp are shown in Table 1. A paper was then made (asdescribed in Comparative Example A) from the pulp and properties of theas-produced paper are shown in Table 2.

The 100% refined wood pulp material as described in Comparative ExampleA results in high CSF values. With the addition of PBO to wood pulp andcorefining, the resulting pulp displays a drop in the CSF values. Thiseffect is clearly observable in Example 2, where 25/75 PBO/woodpulpco-refined pulp was made and is shown in the optical micrograph that isFIG. 6. The CSF value obtained with this sample is dramaticallydifferent from the CSF value obtained by mixing pulp from ComparativeExample A (100% PBO) and B (100% wood pulp) in a 75/25 dry weight basisratio as described in Example 2. These examples demonstrate thebeneficial effect of co-refining wood pulp with PBO versus mixing ofpulp products in conventional mixing equipment.

The fiber length averages of the pulp products produced in the Examplesare listed in Table 1. It is interesting to note that the co-refinedsamples of this invention (Examples 1 and 2) have a shorter fiber lengththan the pulp produced in Comparative Example C. This demonstrates thatby co-refining PBO with wood pulp, very different types of pulp productsare produced that cannot be achieved by merely mixing PBO pulp with woodpulp.

Table 2 summarizes modulus and tenacity results obtained from the handsheet papers made in the Examples. Examples 1 and 2 show most impressiveand surprising data: the papers formed exceed the modulus and tenacityproperties of a pulp made by simply mixing the pulps in a 75/25 ratio(Comparative Example C) and in some cases surpass even thesingle-material papers of Comparative Examples A and B. As ComparativeExample C demonstrates, these results can only be achieved when PBO andwood pulp are processed together. Simply mixing wood and PBO pulp doesnot resolve in any benefit over 100% wood pulp paper.

TABLE 1 Length Arithm. weighted Wt % mean mean Weight wood Wt % CSFlength length weighted mean Pulp pulp PBO [mL] [mm] [mm] length [mm] A100 0 675 0.382 0.891 1.318 B 0 100 670 0.209 1.174 2.691 1 91 9 6920.402 0.907 1.342 2 75 25 498 0.351 0.862 1.448 C 75 25 695 0.404 1.0411.783

TABLE 2 Pulp Wt % Tensile Young's Basis from wood Strength ModulusDensity Weight example pulp Wt % PBO [N/cm] [MPa] [g/cc] [g/m²] A 100 08.12 232.48 0.27 164.17 B 0 100 0.23 1.18 0.23 138.98 1 91 9 7.39 178.110.24 160.50 2 75 25 18.78 419.99 0.26 157.83 C 75 25 5.25 139.93 0.24157.63

Example 3

This example illustrates how the pulp of this invention can beincorporated into a beater-add gasket for fluid sealing applications.Water, rubber, latex, fillers, chemicals, and the pulp of this inventionare combined in desired amounts to form a slurry. On a circulating wiresieve (such as a paper machine screen or wire), the slurry is largelydrained of its water content, is dried in a heating tunnel, and isvulcanized on heated calendar rolls to form a material having a maximumthickness of around 2.0 mm. This material is compressed in a hydraulicpress or two-roll calendar, which increases the density and improvessealability.

Such beater-add gasket materials generally do not have as goodsealability as equivalent compressed-fiber materials and are best suitedfor moderate-pressure high-temperature applications. Beater-add gasketsfind applicability in the making of auxiliary engine gaskets or, afterfurther processing, cylinder head gaskets. For this purpose, thesemi-finished product is laminated onto both sides of a spiked metalsheet and is physically fixed in place by the spikes.

Example 4

This example illustrates how the pulp of this invention can beincorporated into a gasket made by a calendaring process. The sameingredients as in Example 3, minus the water, are thoroughly dry mixedtogether and are then blended with a rubber solution prepared using anappropriate solvent.

After mixing, the compound is then generally conveyed batchwise to aroll calendar. The calendar consists of a small roll that is cooled anda large roll that is heated. The compound is fed and drawn into thecalendar nip by the rotary movement of the two rolls. The compound willadhere and wrap itself around the hot lower roll in layers generallyabout 0.02 mm thick, depending on the pressure, to form a gasketingmaterial made from the built-up compound layers. In so doing, thesolvent evaporates and vulcanization of the elastomer commences.

Once the desired gasketing material thickness is reached, the rolls arestopped and the gasketing material is cut from the hot roll and cutand/or punched to the desired size. No additional pressing or heating isrequired, and the material is ready to perform as a gasket. In thismanner gaskets up to about 7 mm thick can be manufactured. However, mostgaskets made in this manner are much thinner, normally being about 3 mmor less in thickness.

1. A pulp for use as reinforcement or processing material, comprising:(a) fibrillated wood pulp fibrous structures, the structures being 60 to97 weight percent of the total solids; (b) fibrillated polyarenazolefibrous structures being 3 to 40 weight percent of the total solids; thewood pulp and the polyarenazole fibrous structures being withoutsubstantial aggregates or conglomerates of the same material and havingan average maximum dimension of no more than 5 mm, a length-weightedaverage length of 0.7 mm to 1.3 mm, and stalks and fibrils where thewood pulp fibrils and/or stalks are substantially entangled with thepolyarenazole fibrils and/or stalks.
 2. The pulp of claim 1, wherein thewood pulp fibrous structures are about 60 to 75 weight percent of thetotal solids.
 3. The pulp of claim 1, wherein the polyarenazole fibrousstructures are about 25 to 40 weight percent of the total solids.
 4. Thepulp of claim 1 having a Canadian Standard Freeness (CSF) of 100 to 700ml.
 5. The pulp of claim 1, wherein the polyarenazole is a rigid rodpolybenzazole or rigid rod polypyridazole polymer.
 6. The pulp of claim5, wherein the polybenzazole is a polybenzobisoxazole.
 7. The pulp ofclaim 5, wherein the polypyridazole is a polypyridobisimidazole.
 8. Afriction material, comprising: a friction modifier; a binder; and afibrous reinforcement material comprising the pulp of claim
 1. 9. Thefriction material of claim 8, wherein the friction modifier is selectedfrom the group consisting of metal powders, abrasives, lubricants,organic friction modifiers, and mixtures thereof; and the binder isselected from the group consisting of wood pulping resins, melamineresins, epoxy resins and polyimide resins, and mixtures thereof.
 10. Afluid sealing material, comprising: a binder; and a fibrousreinforcement material comprising the pulp of claim
 1. 11. The fluidsealing material of claim 10, wherein the binder is selected from thegroup consisting of nitrile rubber, butadiene rubber, neoprene, styrenebutadiene rubber, nitrile-butadiene rubber, and mixtures thereof.
 12. Apaper comprising the pulp of claim 1 and a binder.